In the oil and gas industry, it is desirable to obtain data from a wellbore. Several real time data systems have been proposed. One involves the use of a physical cable such as an electrical conductor or a fiber optic cable that is secured to the tubular body. The cable may be secured to either the inner or the outer diameter of the pipe. The cable provides a hard wire connection that allows for real-time transmission of data and the immediate evaluation of subsurface conditions. Further, these cables allow for high data transmission rates and the delivery of electrical power directly to downhole sensors.
It has been proposed to place a physical cable along the outside of a casing string during well completion. However, this can be difficult as the placement of wires along a pipe string requires that thousands of feet of cable be carefully unspooled and fed during pipe connection and run-in. Further, the use of hard wires in a well completion requires the installation of a specially-designed well head that includes through-openings for the wires.
Various wireless technologies have been proposed or developed for downhole communications. Such technologies are referred to in the industry as telemetry. Several examples exist where the installation of wires may be either technically difficult or economically impractical. The use of radio transmission may also be impractical or unavailable in cases where radio-activated blasting is occurring, or where the attenuation of radio waves near the tubular body is significant.
The use of acoustic telemetry has also been suggested. Acoustic telemetry employs an acoustic signal generated at or near the bottomhole assembly or bottom of a pipe string. The signal is transmitted through the wellbore pipe, meaning that the pipe becomes the carrier medium for sound waves. Transmitted sound waves are detected by a receiver and converted to electrical signals for analysis.
In the downhole application of acoustic telemetry wireless networks, communications reliability and range are two highly desirable performance issues. While the use of a single piezoelectric transducer with an associated transceiver offers fabrication advantages, design compromises can impact performance. For example, one major drawback of the single transducer/transceiver design is that both transmitter and receiver performance may be compromised in order to accommodate the single transducer design.
Accordingly, a need exists for alternative electro-acoustic communications node designs, for use in wellbore acoustic telemetry systems, which offer improved communications performance.
The use of piezoelectric transducers in a downhole wireless telemetry system presents further challenges. For example, the fabrication and installation processes associated with a piezoelectric transducer can introduce variability of the transmit and receive sensitivities. A need exists for a method to assess the quality of the piezoelectric transducer and its installation in the acoustic telemetry devices at an early stage of the system fabrication process. Additionally, the ceramic crystals comprising piezoelectric transducers are delicate and subject to breakage in rough conditions typical of downhole environments. Moreover, it is critical that the piezoelectric transducers are attached securely to their respective transmission/reception substrates. Typically an epoxy bond is used to secure the transducers and to provide a good acoustic interface, but the epoxy bond is not always reliable in a downhole environment. Furthermore, the epoxy bond introduces undesirable variability into the performance of the transducer. What is needed is an economical, easy to implement method to secure the transducer to its substrate.